On the Frequency Dependence of the PDIV in Twisted Pair Magnet Wire Analogy in Dry Air

This article presents an experimental verification of a partial discharge inception voltage (PDIV) model for twisted-pair magnet wires (TPMWs), which parametrizes the PDIV of wedge-shaped gas gaps (such as TPMWs) solely in terms of its reduced coating thickness—the ratio of coating thickness to its relative permittivity. According to the literature, the coating's permittivity and thickness should play a significant role in determining the PDIV. Still, no explicit quantification of their concurrent effects is often given, and they are frequently treated as decoupled parameters. Therefore, extensive measurements of PDIV of samples analogous to TPMWs coated with dielectric materials of different permittivity [sealed alumina (SA) and unsealed alumina (UA), silicone polyester (SiPo), and perfluoroalkoxy alkane (PFA)] and thickness ( $\approx 45$ – $80 \mu \text{m}$ ) are performed. The PDIV of the test samples is determined under sinusoidal test voltage in the frequency between 10 and 60000 Hz. It is shown that the PDIV can be predicted based on the relative permittivity and thickness of the coating, and the frequency dependence is due only to the change of permittivity with frequency. Temperature and pressure are kept approximately constant at their ambient values (ca. 23 °C and 970 mbar), and only dry air is used to exclude the effect of humidity on the insulation's permittivity. Ultimately, the role of surface charge is discussed qualitatively by comparing the PDIV with the partial discharge extinction voltage (PDEV) and their repetitive counterparts (RPDIV and RPDEV).